The child initiates and creates free play. It might involve fantasies -- such as pretending to be doctors or princesses or playing house -- or it might include mock fighting, as when kids (primarily boys) wrestle and tumble with one another for fun, switching roles periodically so that neither of them always wins. And free play is most similar to play seen in the animal kingdom, suggesting that it has important evolutionary roots. Gordon M. Burghardt, author of The Genesis of Animal Play, spent 18 years observing animals to learn how to define play: it must be repetitive -- an animal that nudges a new object just once is not playing with it -- and it must be voluntary and initiated in a relaxed setting. Animals and children do not play when they are undernourished or in stressful situations. Most essential, the activity should not have an obvious function in the context in which it is observed -- meaning that it has, essentially, no clear goal.